Which came first, the fashion for urban 20-year-olds to wear brogues and waistcoats or the success of rustic British pop? It's hard to tell, since a general brownness has been ubiquitous for the past half-decade in a period when British fashion and music have been cross-pollinating to the gain of both. That might help explain why Mumford & Sons have become one of the beleaguered music industry's good news stories.

Certainly, few would have predicted back in 2006 that Mumford banjo player Winston Marshall's raggle-taggle folk night – Bosun's Locker – would give rise to a clutch of well-regarded artists like Johnny Flynn, Laura Marling and Noah and the Whale. And that one of their number – Mumford & Sons – would, by 2012, have sold over 5m copies of their debut album, Sigh No More, snared a Brit, and become, after Adele, the UK's must-have new musical export. In America, roots music is a huge force that rivals the pop economy; the US clasped this wholesome foursome in a particularly enthusiastic bear-hug, out of which popped two Grammy nominations.

As a result, the follow-up album bears a heavy yoke of expectation, one lightened with the sure bet of M&S's constancy. M&S (and it is an unfortunate abbreviation) are not a band who might suddenly recant their acoustic ways and seek out some dubstep fettling. Babel is the logical and effective successor to Sigh No More; an emotive, faintly God-fearing clutch of songs. SNM producer Markus Dravs (Arcade Fire, Coldplay) reprises his previous role of making the Mumfords sound like a cross between the stomp and clamour of Arcade Fire and Coldplay's brow-furrowed yearning.

Many of these songs – like live favourite Lover of the Light , the banjo-powered lay hymn Whispers in the Dark or the more intriguingly tormented Lover's Eyes – have existed for over 18 months, beaten into studio shape by audience response. It's a sound practice that goes against the grain of most artist management nowadays, which tends to guard new intellectual property closely. Mumford & Sons have rarely been off tour, a laudable dedication to the troubadour's craft which has extended to hosting their own festivals, Gentlemen of the Road. You cannot fault their love for playing out-of-the-way places , their imperative to keep things human-sized and people-powered or their wariness of a rapacious and crass mainstream. You would happily knit your own ale with them.

For all these brownie points, Babel remains an anodyne record, lacking the shivery authority of Laura Marling's work. Folk is a malleable resource, and here it is stripped of all politics or witness-bearing, becoming an exercise in romantic exegesis for nice men with mandolins. Given heft by Biblical references and lifts from novels, Marcus Mumford's internal weather is intriguing, given his past relationship with Marling and, now, marriage to actor Carey Mulligan, and this second album is less over-weeningly jolly than the first, with incursions of electric guitar and drums.

Babel – track and album – aims to rouse. But with rustic indie in general, and here in particular, great emotion is mostly conveyed by strumming very fast or fiddling very hard; it's the musical equivalent of shouting at foreigners and simpletons. With every crescendo of catgut and steel, their lack of nuance becomes wearing.